The Producer and Creator metadata in the PDF show:
Producer: Mac OS X 10.6.7 Quartz PDFContext
It is clear that the document was last touched by Preview. The only workflow which explains both tags requires one to select Print to PDF. If you save the attachment from your email, there are no changes to the tags, if you open in preview and save, only the producer changes.
1. Save attachment – Producer and Creator metadata remain Xerox Workcentre
2. Open in Preview and save – Producer is replaced by Quartz, creator remains Xerox
3. Open in preview and print to PDF – All metadata now matches